Due to persistent security threats and the possibility of terrorist activities, there is a need for deploying high speed, high resolution, and accurate screening devices at places that are likely targets of such activities. In addition, there exists a requirement for screening of baggage and other items for explosives, contraband and other illicit materials. This requires a screening system which is capable of discriminating between different materials based on one or more unique features of each material such as effective atomic number, chemical structure, physical density, among other variables.
Cabinet X-ray scanners are capable of performing automated threat detection on articles of baggage and divested items by calculating physical properties of objects from the two-dimensional image generated. For example, material density can be estimated from at least two co-planar projection views. Z-effective can be estimated from at least two overlapping projection views acquired at different energy levels. Thus, increasing the number of views or number of energy levels permits for more precise estimations of physical properties such as material density and Z-effective.
Conventional cabinet X-ray systems have a limited number of co-planar views. It is highly desirable for commercial reasons to have as few views as necessary. Typically, a planar X-ray view consists of an X-ray generator and a linear array of X-ray detectors, which constitutes a majority of the component costs in an X-ray scanner. Each additional view increases the component cost of the scanner in an incremental manner. Thus, two views would imply twice the cost of a single view, three views would increase the cost three times, and so forth. Therefore, achieving desired imaging or automated detection performance with the fewest number of views allows the lowest component cost. Increasing performance through other means, such as higher performance detectors, becomes desirable because of the potential lower increase in component cost.
Further, increasing energy discrimination also has a detrimental impact on component cost. Commercially available X-ray detectors currently permit up to 128 channels of energy discrimination. Populating a multiple or even a single projection X-ray detector array with energy discriminating detectors is commercially prohibitive. The cost of energy discriminating detectors is significantly higher than conventional dual-energy detector arrays. For example, a conventional single-view X-ray scanner equipped with energy discriminating detectors would incur a three-fold increase in component cost. Reducing the number of energy discriminating detectors employed to increase performance becomes an attractive option, especially if the incremental increase in component cost is less than doubling the cost of the machine, i.e. less than the cost of adding a second view.
For an application such as liquids screening, it is desirable to have the full range of energy information permitting for a spectroscopic analysis of the contents in the divested container. Therefore, some applications present the opportunity to restrict the concept of operations such that only a portion of the projection view needs to be populated with multi-energy discriminating detectors in order to obtain improved estimation of the physical properties necessary for effective threat detection. For example, with respect to the divestiture of liquid containers at an aviation security checkpoint for separate threat detection analysis, only a portion of a bin or container need be screened using multi-energy discriminating detectors.
Thus, what is needed is an X-ray scanner, having at least a single-view, with limited energy discriminating detector coverage that can meet or exceed the automated detection performance of a dual-view X-ray scanner and still have a lower component cost, thus achieving the trade-off between cost and performance.
Accordingly, there is a need for an X-ray system that has an overall improved energy resolution to discriminate and therefore detect certain materials of interest.